There are a variety of methods which have been used previously to produce branched polyvinyl aromatics using free radical polymerization. These include the addition of a divinyl monomer, such as divinylbenzene, to the polymerization mixture as described in Macromolecules. 1993, 26, pg. 3131; the use of a multifunctional initiator, such as 2,2-bis(4,4-di-t-butylperoxycyclohexyl)propane as described in U.S. Pat. No. 5,191,040; and the use of a vinyl functional initiator, such as n-butyl-t-butylperoxyfumarate as described in U.S. Pat. No. 4,376,847. However, in these methods branching occurs in the polymerization reactor, causing gels to form. Gels build up in the polymerization reactor after extended periods of continuous operation and lead to reactor fouling.
One approach to solving the problem of polymerization reactor fouling is to place latent functionality on the polymer during polymerization, which subsequently reacts to form branches upon thermolysis at temperatures above 200.degree. C., as described in "Incorporation of Benzocyclobutene into Polystyrene Allowing Postpolymerizer Chain Extension/Branching", Macromolecules, 1994, 27, pgs. 1307-1312. In this process the benzocyclobutene (BCB) is incorporated into the polymer chain by initiation with a BCB functional peroxide, resulting in chain extension or branching upon heating at 240.degree. C. However, this approach produces an unstable polymer since the branching of the BCB functionalities is incomplete and can continue upon further thermal processing, thus changing the degree of branching, the molecular weight and polymer properties.
Accordingly, it would be advantageous to produce a stable branched vinyl aromatic polymer by a continuous method that would overcome the foregoing disadvantages of the prior art.